Method and apparatus for the semi-continuous melting and discharging of ceramic material in an induction melting furnace with sintering crust crucible

ABSTRACT

A method is disclosed for the semi-continuous melting of ceramic material by means of inductive melting in high-frequency and medium-frequency induction melting furnaces whose melting coil surrounds a sintering crust crucible and contains a run-out channel. In tile method, melt is periodically run out and material to be melted is supplied to the crucible so as to replace the material removed preferably so as to maintain a constant level. An intensively cooled channel is used as a run-out device. For the melt broaching, the melt nose is grasped from below by way of an automatically controllable broaching lance of a broaching device and raised. The broaching lance is then advanced between the bottom of the channel and the solidified melt until the sintering crust is pierced. The method permits a reliable and economic management of the process and the maintenance of the quality of the melted products. A device for periodic melt broaching as well as an induction melting furnace equipped with an intensively coolable run-out channel provides apparatus for carrying out the above method.

This is a divisional of application Ser. No. 07/842,606 filed on Feb.27, 1992, now U.S. Pat. No. 5,268,925.

FIELD OF THE INVENTION

The invention is relative to a method of semi-continuous melting anddischarging of ceramic materials in high-frequency and medium-frequencyinduction melting furnaces with sintering crust crucible wherein melt isremoved by means of a periodic melt breaching. Further subject matter isrelative to a furnace suitable for use in the present method and to adevice for carrying out the periodic melt breaching. This devicecomprises a tool with a movable broaching lance for removing solidifiedmelt in the run-out channel of the furnace and the piercing of thesintered crust.

BACKGROUND DISCUSSION

Induction melting furnaces are known in various designs and thefrequencies which can be used extend from approximately 50 Hz into theMHz range. The design of the furnace and crucible for receiving the meltas well as the devices for removing the melt depend to a considerableextent on the thermal and electric properties of the melt material. Inorder to melt ceramic materials with melting points above 1000° C.,especially between 2000° C. and 3000° C. and to avoid impurities of themelt by the crucible material, the so-called skull or sintering crusttechnology has proven to be suitable, in which the material to be meltedforms a sintering crust crucible on the cold wall of a crucibleform--cf. W. Assmus, "Chemie Ingenieur Technik" 55, (1983) (9), pp.716-707 and V. J. Alexanderov, "Current Topics in Materials Science"Vol. 1, (1978), pp. 421-280.

The technology described in the cited documents and the crucibles usedtherein do not permit a semi-continuous production of a high-meltingmaterial with external further processing of the melt. Moreover, thistechnology exhibits further disadvantages as it is time-consuming andwork-intensive on account of the particular refilling of the crucible,formation of the sintering crust and cooling off. In additions,fluctuations in quality from batch to batch can not be excluded.

The high-frequency induction melting furnace of French Patent No.1,430,192 permits the attainment of temperatures of up to 3000° C. andhas a sintering crust crucible. The crucible according to FIG. 2 of theFrench patent comprises a spout for teeming the melt. The entirecontents of the crucible can be emptied herewith by tilting in as far asa sintering crust optionally formed on the surface of the crucible ispierced through. No suggestion can be gathered from this document for asemi-continuous operation by removing a part of the melt at periodicintervals and supplementing the removed amount of melt by periodicallyor continuously adding material to be melted or for a suitable device tothis end. A total emptying of the crucible is disadvantageous becausethe total refilling of the crucible with the dielectric, powderymaterial to be melted as well as with the mental required for theinitial energy absorption which is then necessary is expensive andentails a low space-time yield. In addition, the further processing ofthe total molten crucible contents results in problems on account of theamount of energy expended if the crucible contents are large. Finally,rather large variations in the quality of the individual cruciblecharges are difficult to avoid.

EP-A 0,079,266 teaches a further induction melting furnace with asintering crust crucible. This furnace with several windings is operatedwith a frequency of preferably 10 kHz to 50 kHz and comprises a devicefor removing the melt at the bottom of the furnace. This .devicecomprises a discharge tube which extends through the bottom and issurrounded by an inductor which is independent of the actual furnace.The discharge tube consists of a material which absorbs the selectedfrequency and can be closed with a plug of the same material. Thisdischarge device, which must not enter into any conductive connectionwith the bottom of the crucible, must be cooled during the actualmelting process by a suitable cooling device. The technical design ofsuch a device is therefore very expensive. Moreover, the introduction ofimpurities into the molten material due to contact with the material ofthe discharge tube can not be sufficiently excluded, especially if, forexample, the melt is to be discharged periodically or even continuously.The furnace of EP-A 0,079,266 is suitable for melting those ceramicmaterials whose so-called coupling temperature and melting temperatureare very close to one another. In contrast thereto, short circuits andassociated damage to the inductor and the generator can occur in thecase of materials with a coupling temperature and a melting temperaturewhich are far apart from one another. The furnace in EP-A 0,079,266 canthus be used only in a limited fashion, e.g. for melting glass andenamel but not very high-melting materials such as e.g. titanium dioxideor zirconium silicate.

EP-B 0,119,877 teaches a high-frequency induction melting furnace whosewall simultaneously forms the inductor and the cold wall side of asintering crust crucible. The inductor includes a single flat windingwith several conduits. According to one embodiment, the furnacecomprises an optionally cooled tube passing laterally through the coilwhich tube is intended to remove the melt. In the case of materials witha high melting point and especially in the case of materials such aszircon sand, in which a volumetric increase occurs during cooling off,e.g. by means of modification transformation, such a tube becomesclogged; however, a heating of the tube can not be effected and/orresults in considerable technical expense and/or material problems.

An attempt by the Applicant of the present invention at simultaneouslysupplying material to be melted and removal of the melt in a furnace ofthe above-mentioned type (EP-B 0,119,877) proved to be impracticable inthe case of high-melting materials such as zircon sand because eitherthe level drops too rapidly and non-molten material is entrained by themelt or, in the case of too slight a flow-off, the melt solidifies inthe run-out tube. An opening of the run-out tube proved to be extremelyproblematic. The high resistance of the solidified melt in the run-outtube aggravated or prevented an opening by means of knocking with chiseland hammer. Even drilling out the solidified material presenteddifficulties on account of the hardness and brittleness of the materialsexamined. In the case of only a partial opening of the tube, it closedup rapidly again due to crust formation. In addition, manual labor inthe area of a high- to medium-frequency scattered radiation is notadmissible and is dangerous even after the oscillating circuit has beenshut off on account of the hot melt.

EP-A 0,248,727 describes a generic induction melting furnace whichcomprises only a single flat winding with one conduit as the inductor.Customary tube generators are used for the high-frequency range andsemiconductor generators are used for the medium-frequency range.According to one embodiment of the furnace in EP-A 0,248,727, thefurnace is provided from below with a bottom outlet tube with a plug.This device permits only a total emptying of the crucible contents withthe disadvantages already mentioned.

A total or optionally partial emptying of an induction melting furnaceby means of tilting the furnace by means of a tilting device and teemingthe melt via a teeming lip is known from U.S. Pat. No. 2,785,214.However, the teeming is only possible in as far as no thermal radiationcrust from the material to be melted has formed over the melt. However,such a crust is desirable from an energy saving standpoint.

SUMMARY OF THE INVENTION

The present invention is thus directed at providing a method which makespossible a semi-continuous melting and discharging operation inaccordance with sintering crust technology in induction melting furnaceswith a sintering crust crucible. The method is directed at permitting aperiodic removal of a defined amount of melt from the crucible whileavoiding the disadvantages of the previously known methods. The presentinvention makes use of an induction melting furnace suitable forperiodic melt broaching as well as a device for the periodic melt broachjug of the same which can be automatically controlled.

A method was found for the semi-continuous melting of ceramic materialby means of inductively melting it in the high- and medium-frequencyrange in induction melting furnaces which each include a meltinginductor coil structure defining a wall that surrounds a sintering crustcrucible, discharging the melt through a run-out device extending fromthe melting inductor coil structure and charging the furnace with anamount of the material to be melted which corresponds to the amount ofmelt discharged (e.g., supplying material which, when melted, representsthe same amount of melted material discharged).

The method of the present invention is further characterized in that anopen and intensively cooled run-out channel is located at the upper edgeof the melting inductor and is used as the run-out device. The run-outof the melt is opened in periodic intervals by means of broaching themelt with a broaching device. The broaching device comprises a broachinglance and means for automatically regulating the position of thebroaching lance. The automatic regulating device varies the angle ofinclination of the broaching device for a parallel shifting of thebroaching lance, which is positioned horizontally or at an incline. Thelance is preferably positioned so as to extend essentially parallel tothe bottom of the run-out channel. The automatic regulating devicedrives the lance such that the broaching lance is first guided under themelt nose of the remainder of solidified melt remaining in the run-outchannel from the preceding melt broaching, is then raised with this noseand then subsequently driven forward between the raised, solidified meltand the bottom of the channel until it pierces through the sinteringcrust. The method is also characterized in that the amount of run-out isregulated as required by tilting the furnace by means of a tiltingdevice.

The method of the present invention and the related apparatus includingthe furnace, coolable run-out channel, and the device for periodic meltbroaching are suitable for melting ceramic materials which form asintering crust crucible under the aforementioned operating conditions.Operating temperatures for operating conditions like those describedabove are, in general, above 1000° C., especially in a range of 1500° to3000° C. The concept "melt" includes chemical reactions in the meltedstate.

The term "semi-continuous" means that approximately to 70% of the melt,especially 5 to 30% of the crucible contents, is let out of the crucibleat a time in periodic intervals and after each discharge thecorresponding amount of the material to be melted is resupplied andmelted. The amount of melt allowing out per broaching is a function ofthe geometry and the degree of charging of the crucible, of the form andposition of the run-out channel and of the angle of inclination of thecrucible. The angle of inclination of the crucible and surroundingmelting inductor structure can be freely selected via a tilting devicebetween 0° and 90° and the inductor structure is usually tilted duringthe periodic broaching only in a range between 0° and 30°.

BRIEF DESCRIPTION OF THE DRAWINGS

The method of semi-continuously melting and discharging the melt, theinduction furnace with attached run-out used in carrying out thismethod, and the device for periodic melt broaching are explained usingthe following drawings:

FIG. 1 shows a section through the induction melting furnace inaccordance with the invention with sintering crust crucible and run-outchannel along with the device of the invention for periodic meltbroaching, as well as devices for dosing the material to be melted, forcooling off the melt outside of the furnace and devices for raising andlowering and tilting the furnace. The capacitors of the oscillatingcircuit as well as the generator and the electric devices for energysupply and control of the process are those commonly used in theindustry and are not shown in FIG. 1.

FIG. 2 shows the open, coolable run-out channel.

FIG. 3 includes the initial position as well as three operatingpositions of a device for the periodic melt broaching with respect tothe run-out channel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The principle design of an induction melting system comprises apreferably self-controlled oscillating circuit of an aperiodicgenerator. The oscillating circuit is formed by the melting coil of theinduction furnace and by capacitors. The necessary frequency isgenerated in the high-frequency range (greater than 10 kHz to 500 kHz)by tube generators and in the medium-frequency range (around and lessthan 10 kHz to approximately 500 Hz) by semiconductor generators.

FIG. 1 shows a preferred embodiment of an induction melting furnace.Reference number 1 signifies a main melting coil structure which issurrounded by cooling device 2 (e.g. half tubes welded onto the coil ora double walled jacket, as is indicated in FIG. 1) which surroundssintering crust crucible 3. Sintering crust crucible 3 is comprised ofthe material to be melted, and receives melt 4 therein. The melting coilrests on cast form 5, which also receives cooled coil bottom 5a isolatedfrom the main melting coil structure. Open run-out channel 6 is fixed inthe area of the upper edge of the main coil structure by means ofholding device 6/4 shown in FIG. 2 and bores 6/5 for fastening elements(not shown). Channel 6 is fixed in position such that the channel bottomis located before the broaching of the melt, below the level of the meltand such that the upper edge of the channel essentially corresponds withthe edge of the main melting coil structure. The cross section of thechannel as well as its inclination influence the amount of melt runoff.A slight inclination or the channel and a small difference in heightbetween the channel bottom on the furnace-side end and the level off themelt in the crucible can result, on account of the slow flow, in a rapidsolidification of the melt in the channel and in the formation of a newsintering crust on the furnace-side end of the channel.

The melting coil structure can have one or several windings althoughsingle-winding coils of copper or aluminum are preferred. In order tofacilitate the removal of the melt regulus after the end of the meltingwith periodic melt broachings and cooling-off, it is advantageous toemploy a coil body with a slightly conical form (according to FIG. 1) ifthe melt material expands when cooling off, e.g. due to modificationtransformations.

The melt crucible is charged from storage container 10, from which thematerial to be melted is supplied to a device 9 for the gravitationaldosing of material 8 over essentially the entire surface area of thecrucible opening in an essentially equally dispersed manner. In order tominimize heat losses, thermal protection shields 7 can be arrangedaround the upper edge of the furnace. In addition, it is advantageous tokeep the crucible surface covered with the material to be melted.

The solidified regulus can be readily removed from the coil by means ofa device 23 for raising and lowering the coil bottom 5a. As shown inFIG. 1, device 23 comprises lift cylinder 24 and drive unit 25. The maincoil structure and crucible contained therein are tilted by means oftilting device 27, which can be designed in various forms. In FIG. 1,reference number 28 designates a lift rod, 29 a drive unit, 30 and 31represents points of attachment between the lifting device and 26represents the point of rotation of the furnace. The amount of meltoutflow can be regulated by regulating the tilting of the furnace.

Device 32 serves to cool off the melt running out periodically viachannel 6.

The device in accordance with the invention for periodic melt broachingcomprises at least one lifting device (16, 21) for varying theinclination or for the vertical parallel shifting of the advancinglinkage rods (19) carrying or supporting broaching lance (20). Thedevice comprises at least one adjusting device (14, 15) for advancingthe advancing linkage with attached broaching lance. Also, the liftingdevice and adjusting means can be regulated in a programmable mannerusing programmable regulating means. FIG. 1 illustrates programmableregulating means 37 which includes automatic control means 33 andactuation communication lines 34, 35, 36 in communicating with liftingdevice 16 and adjusting means 14, 15.

In the preferred embodiment of the melt-broaching device shown in FIG.1, the lifting device is designed as a cylinder assembly comprised ofpneumatic cylinder 16 with lifting rod 21. Cylinder 16 has one end fixedin a pivotable manner to fastening point 17 of holder 11. Lifting rod 21is attached at its free end to shackle 12 at pivot point 18. Shackle 12receives advancing linkage 19 and is rotatably fastened to holder 11 atfastening point 13. The angle of inclination of broaching lance 20 canbe varied in a vertical direction by activating lift cylinder 16 and canbe adapted therewith to the angle of inclination of run-out channel 6.The adjusting device for advancing or retracting linkage 19 and attachedbroaching lance 20 is formed by two pneumatic cylinders 14,15 which arefastened to holding shackle 12 and adapted to move linkage 19 in atelescopic fashion. Instead of pneumatic cylinders, other means with thesame function, that is, raising/lowering and advancing/withdrawing, suchas e.g. spindles can also be used. An alternative lifting devicesuitable for use in the present invention includes a drive device incontact with holder 11, which in this instance contains the broachinglance and devices for advancing, that is adapted to raise and lower orrotate holder 11 with respect to support 22. According to a preferredembodiment, broaching lance 20 is designed as a drill bit which executesa fixed-cycle forward movement upon a predetermined amount ofresistance, similar to a hammer drill, with or without rotary motion.

FIG. 2 shows a preferred embodiment of the coolable channel, which isopen on top. The channel jacket contains a maximum number of bores 6/1extending parallel to the longitudinal direction of the channel (shownin dotted lines in FIG. 2) which are connected to each other in ameandering fashion and through which a cooling medium flows which issupplied through line 612 and removed through line 6/3. Holder 614 isfixed to the melting coil structure by means of fastening elements (notshown) extending through holes 6/5 and the coil structure. Holder 6/4provides a way to position the channel. As intensive a cooling of thechannel as possible is preferred because under this condition no cakingsoccur in the melting operation aside from a readily removable melt nose.

FIG. 3 shows the initial position and three operating positions of thebroaching tool together with the open-ended channel, which also containssolidified melt 4/2 in positions 1 to 3. The broaching lance is firstbrought under melt projection or nose 4/1 by means of aprogram-controlled actuation of cylinder 14 and therewith by the advanceof linkage 19. In a second stage, lifting cylinder 16 is actuated, as aresult off which the broaching lance is upwardly inclined and the meltnose raised thereby (position 2). Finally, cylinder 15 is actuated andthe lance is advance under the solidified melt on the bottom of thechannel (position 3), as a result of which the solidified melt iscompletely raised and the sintering crust is easily pierced on thefurnace-side end of the channel.

In the case of an especially thick sintering crust, it can beadvantageous to broach the sintering crust with a second broaching lancein the area of the transition from the melting coil wall to the channelin addition to the broach lag with the device of the invention.

While the melt is running off, the crucible can be loaded further withraw material if a subsequent separation of the molten and non-moltenmaterial entrained during the runoff does not pose any problems.

It could not have been predicted that the guiding in accordance with theinvention of the broaching lance by means of the device conceived tothis end permits a melt broaching which is free of problems and whichcan be executed with a low expenditure of energy. Furthermore, it wassurprising that as the intensity of the cooling off the run-out channelincreases, the melt solidified in the channel can be more readilyremoved and, in addition, the amount of solidified material decreases.The following result as significant advantages of the method of theinvention and off the devices for carrying out this method:

Problem-free melt broaching, which makes a trouble free, semi-continuousoperation possible.

Reliable management of the melting technology even on a manufacturingscale.

The ability to use medium-frequency induction melting furnaces withsingle-winding melting coils with a rather large diameter andcorrespondingly large crucible contents.

Uniform quality or the melted products by means of a semi-continuousoperation instead of the batch operation, which was considered in thepast to be hardly avoidable in the case of very high-melting materials.

An increase of the space-time yield because repetitive, time-consumingmeasures for refilling the crucible, cooling off and removing the meltregulus are necessary only at rather great time intervals.

An increase in the work satiety of the personnel working with themelting system on account of the possibility of using the automaticallycontrolled broaching device.

EXAMPLES

Zircon sand was melted at approximately 2700° C. in an induction meltingsystem according to FIG. 1 and using melting coils of differing geometryto each off which a run-out channel in accordance with FIG. 2 was fixedto the upper melting coil structure edge and the illustrated meltbroaching device for periodic melt broaching was used. The periodic meltbroaching took place in accordance with FIG. 3. The melt was quenched ina quenching channel with compressed-air nozzles and water nozzles, whichresulted in the obtention of a zirconium dioxide--silica mixture in theform of granules. The nozzles were positioned in serial fashion alongthe inside vertical edge of container 32.

The melting crucible was filled to approximately 90% with zircon sand atthe start of the test. In order to absorb the high-frequency field belowthe coupling temperature of the zircon sand and to heat the zircon sandby thermal contact, horizontal molybdenum platelets were embedded in thezircon sand. After the crucible contents had been thoroughly moltenexcept for a sintering crust, zircon sand was added in and thesemi-continuous operation begun. The test parameters and results followfrom the table.

    ______________________________________                                        Example        1      2      3    4    5    6                                 ______________________________________                                        Melting coil:                                                                 Diameter (cm)  25     40     40   40   60   90                                Height (cm)    25     25     25   25   30   45                                Number of windings                                                                            2      2      1    1    1    1                                Operating frequency (kHz)                                                                    150    120    250  250  30   10                                (Examples 1 to 5)                                                             tube generator                                                                (Example 6                                                                    semiconductor generator)                                                      Run-out channel:                                                              Width (mm)     16     16     16   16   16   22                                Cooling water (l/h)                                                                          400    400    400  400  400  400                               Throughput (kg ZrSiO.sub.4 /h)                                                               16     20     29   33   52   180                               Number of broachings                                                                         4-6    4-5    2-3   1    3    3                                per hour                                                                      Operating time (h)                                                                           40     40     100  20    8    8                                ______________________________________                                    

The melt broachings took place without problems because the melt of thepreceding broaching, which had solidified in the channel, was able to beraised together with the melt nose by means of the broaching device,which freed the way for the piercing of the sintering crust. Asignificant reduction of the volumetric flow of cooling water throughthe cooling device of the channel resulted immediately in crusts whichwere difficult to remove, as a result of which the operation of thesystem was considerably disturbed.

Further variations and modifications will be apparent to those skilledin the art and are intended to be encompassed by the claims appendedhereto.

German Priority Application No. P 41 06 537.9 is relied on andincorporated by reference.

What is claimed is:
 1. An apparatus for a periodic melt broaching of aninduction melting furnace with sintering crust crucible, comprising:abroaching lance; means for supporting said broaching lance, said meansfor supporting said broaching lance including means for angularly andvertically adjusting a position of said broaching lance; means forshifting said broaching lance between a retracted position and anextending position; and wherein said supporting means for supportingsaid broaching lance includes a holder, and a cylinder assembly withlift rod and a first and a second end, said first end of said cylinderassembly being pivotably attached to said holder and the second end ofsaid cylinder assembly being attached to a shackle, said shackle beingpivotably attached to said holder, a linkage device telescopicallyreceived by said shackle, said linkage device including means forengaging said broaching lance, and said means for shifting beingsupported by said supporting means and extending between said shackleand said linkage device.
 2. An apparatus as recited in claim 1 furthercomprising means for regulating said means for angularly and verticallyadjusting and said means for shifting.
 3. An apparatus as recited inclaim 1 wherein said means for shifting includes a pneumatic deviceadapted to extend said linkage device and said broaching lance away fromsaid shackle and to draw said linking device and attached lance closerto said shackle.
 4. An apparatus as recited in claim 1, wherein saidbroaching lance includes a drill bit.
 5. An apparatus as recited inclaim 2, wherein said means for regulating includes automatic controlmeans for regulating.